Stairstep waveform generator



April 24, 1962 H. E. MURPHY 3,031,583

STAIRSTEP WAVEFORM GENERATOR Filed April 14, 1960 Ala/V0574 54 E All/4 7714564 702 FIG.2.

HOWARD E. MURPHY INVENTOR.

ATTORNEYS.

Unite States atent O 3,031,583 STAIRSTEP WAVEFORM GENERATOR Howard E. Murphy, Redwood City, Calif., assignor to This invention relates to stairstep waveform generators and, more particularly, to improvements therein.

A stairstep waveform generator is, as its name implies, an electrical circuit which can provide an output signal whose voltage waveform resembles stairsteps. This waveform can be a voltage which increases to a first amplitude, maintains this amplitude for a given predetermined interval, then increases to a second and higher amplitude, which also is maintained for a predetermined interval, etc. A successive increase to new plateaus occurs until the highest plateau is reached, at which time the voltage amplitude is dropped to the starting level. Waveforms of this type may be used, for example, as a deflection signal for a cathode-ray tube device being used to display alpha-numeric characters, and the width of the displayed characters may be made relatively independent of the character rate.

In the prior art, the stairstep waveform similar to the one briefly described above could be generated by adding two linear sawtooth waveforms having appropriate frequencies and amplitudes. However, this resulted in a system which is relatively complex in that there is involved the generation and addition of two separate linear sawtooth signals.

An object of this invention is the provision of a relatively simple stairstep waveform generator circuit.

Another object of this invention is the provision of a novel and inexpensive stairstep waveform generator circuit.

Still another object of the present invention is the provision of a stairstep waveform generator circuit which generates an output voltage waveform which has linearly sloping risers which have a preselectable duration and which do not depend upon either the character of the input signal wave shape, its amplitude, or its duration within broad limits.

These and other objects of the present invention may be achieved by an arrangement wherein a first transistor is biased to be conductive in saturation. A second transistor is connected to the first transistor in a manner to be held nonconductive as long as said first transistor is conductive. Means are provided for applying a signal to the first transistor to render it nonconductive, whereby the second transistor can become conductive. A capacitor is in series with the second transistor and as a result when the second transistor becomes conductive the capacitor is charged up by the current flowing through that second transistor. A Zener diode is connected to the second transistor in a manner to maintain the level of the charging current flowing through the second transistor constant. A third transistor is connected across the capacitor. The third transistor is normally maintained nonconductive. Upon the application of a conductive signal to the third transistor, it can conduct and discharge the capacitor. A stairstep waveform output is derived from the capacitor.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as Well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is a drawing of a linear stairstep waveform; and

FIGURE 2 is a circuit diagram of an embodiment of the invention.

Reference is now made to FIGURE 1 of the drawings, which illustrates a stairstep waveform of the type which may be generated by the embodiment of this invention. The stairstep waveform comprises linearly rising segments 10, 10', 10", 10", separated by plateaus 12, '12, 12", 12". From the last of these plateaus, the retrace portion 14 of the curve may be seen, which indicates that the level of the wave shape is reduced to its initial value again. The linearly rising waveform segments 10, 10, 10", 10 are obtained by allowing a constantcurrent generator to charge a fixed capacitor for the duration of a step signal input. During the plateau portions 12, 12', 12", 12 the constant-current generator is cut off so that the capacitor charge remains at constant potential. The duration of the plateau is variable and is determined by the duration and rate of the step input signal. The capacitor is discharged by a current switch during the retrace time.

Referring now to FIGURE 2, which constitutes a circuit diagram of an embodiment of the invention, there may be seen a first transistor 20, which is of the PNP type and which has a base 20B and emitter 20E and a collector 20C. There is alsoprovided a second transistor 22, which has a base 22B, an emitter 22E, and a collector 22C. This second transistor is of the NPN type. A third transistor 24, which is of the PNP type, has a base 24B, an emitter 24E, and a collector 24C. Operating potential for these transistors is provided by a source of potential (not shown), which includes a negative potential output terminal 26, a positive potential output terminal 28, and a ground potential output terminal 30.

A first bias resistor 32 is connected between the negative potential output terminal 26 and the base 20B of the transistor 20. The emitter 20E is connected to the ground potential terminal 30. The collector 200 is connected to the negative-potential terminal 26 through a second bias resistor 34, as is also the emitter 22E of the transistor 22. The collector 22C of the transistor 22 is connected to the ground-potential terminal 30 through a capacitor 32. A Zener diode 34 is connected between the negative-potential terminal and the base 22B of transistor 22. A resistor 36 connects the base 223 to the terminal 30. The third transistor 24 is connected across the capacitor 32. Its collector is connected to the collector 22C and its emitter 24E is connected to the terminal 30. A third bias resistor 38 is connected between the base 24B and the positive potential terminal 28. A resistor 40 connected between the terminals 28 and 30 isolates these terminals. The potential applied to the base of the transistor 24 is more positive than that applied to the emitter; therefore, this transistor is normally maintained not conductive. A capacitor 40 connects the base to a terminal 42, to which signals may be applied for rendering the transistor 24 conductive when desired. A terminal 44 is connected to the capacitor 32, whereby between terminal 44 and ground-potential terminal 30 the output stairstep waveform of the circuit may be derived. A capacitor 46 connects the base 20B of the transistor 20 to an input terminal 48.

Transistor 20, in the absence of an input signal on terminal 48, is maintained conductive in the saturation state. The bias is derived through resistor 32, which applies a more negative potential to the base [20B than is applied to the emitter 20E. The collector current drawn by transistor 20 through resistor 34 biases off transistor 22. The emitter 22E is maintained more negative than the base 223 by virtue of the collector current being drawn through the resistors 20, 34. Upon the application of a signal to the input terminal 48 by a source which is preferably a monostable multivibrator 5t), transistor 20 is rendered nonconductive, whereby the collector current no longer flows through the resistor 34. This places a large negative signal on the emitter 22E, causing transistor 22 to become conductive. The emitter-to-collector current charges the capacitor 32 during the time that the input signal maintains the transistor 20 nonconductive. This charging period corresponds to one of the intervals 10, 10", 10", shown in FIG- URE 1.

The current flowing through the transistor is maintained constant by virtue of the regulatory action of the Zener diode 34, which in conjunction with the series resistor 36 maintains a constant current flowing through the serial combination, and thereby a constant-voltage signal applied to the base 22B. As a result, the rising voltages are linear with respect to time. When the input signal is turned ofi, the voltage across the capacitor 32 remains constant, since there is no way for the capacitor to discharge except through transistor 24, which is maintained nonconductive. Thus, the plateau intervals 12, 12', 12", 12' are provided for. Upon the application of a signal to the terminal 42, transistor 24 is rendered conductive, and since it is connected across the capacitor 32 it discharges the capacitor during the retrace interval 14.

A monostable multivibrator 50, as previously indicated, is a preferred input-signal source. The monostable multivibrator is driven by any suitable signal applied to its input. Regardless of the amplitude of that signal, provided that it is enough to cause the monostable multivibrator to be driven, the output from the multivibrator is a standard signal. Thus, it serves the function of standardizing the input signal so that the duration of the rising voltage segments 10, 10', 10", 10", are preselectable by varying the time constants of the monostable multivibrator, and do not depend upon the character of the input signal wave shape, its amplitude or duration within extremely broad limits.

There has accordingly been shown and described herein a novel, useful, simple, and inexpensive stairstep waveform generator. The rising portions of the waveform may be made linearly variable with time, and the plateau intervals may be determined by the duration and rate of the input signal which drives the monostable multivibrator.

I claim:

'1. A stairstep waveform generator comprising first, sec- 0nd, and third transistors each having collector, emitter, and base electrodes, bias means connected to said first transistor base for maintaining it conductive in saturation, impedance means connected to said first transistor collector and to said second transistor emitter for maintaining said second transistor nonconductive while said first transistor is conductive, a capacitor, means connecting said capacitor in series with the collector of said second transistor to be charged by current passing through said second transistor, a Zener diode, and means to connect said Zener diode to said second transistor base to regulate the current flowing through said second transistor means for applying a signal to said first transistor base to render said first transistor nonconductive responsive thereto, means connecting said third transistor collector and emitter respectively across said capacitor, impedance means connected to said third transistor base to maintain said third transistor cut off, means for applying a signal to the base of said third transistor to render it conductive for discharging said capacitor, and terminal means connected to said capacitor for deriving an output waveform therefrom.

2. A stairstep waveform generator as recited in claim 1 wherein said first and third transistors are of the PNP type and said second transistor is of the NPN type.

3. A stairstep waveform generator comprising first, second, and third transistors each having collector, emitter, and base electrodes, said first and third transistors being of the PNP type, said second transistor being of the NPN type, a source of operating potential including a negativepotential output terminal, a positive-potential output terminal and a ground-potential output terminal, a first bias resistor connecting said first transistor base to said negative-potential output terminal, a second bias resistor connecting said first transistor collector and said second transistor emitter to said negative-potential output terminal, means connecting said first transistor emitter to said ground-potential output terminal, a capacitor connected between said second transistor collector and said groundpotential output terminal, a Zener diode connected between said negative-potential output terminal and said second transistor base, a third bias resistor connected between said second transistor base and said ground-potential output terminal, means connecting said third transistor collector to' said second transistor collector, means connecting said third transistor emitter to said groundpotential output terminal, a resistor connected between said third transistor base and said positive-potential output terminal, means for applying an input signal to said first transistor base to render said first transistor nonconductive whereby said second transistor may become conductive and charge up said capacitor, means for applying an input signal to said third transistor base to render said third transistor conductive to discharge said capacitor, and means connected to said capacitor to derive an output Waveform.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,425 Geiger et al. Apr. 8, 1941 2,474,040 Day June 21, 1949 2,563,816 Butman Aug. 14, 1951 2,602,151 Carbrey July 1, 1952 2,693,572 Chase Nov. 2, 1954 2,743,438 Page Apr. 24, 1956 2,796,314 Bishop et a1. June 18, 1957 2,871,378 Lohman Jan. 27, 1959 

